Remote emissions sensing systems generally are known. One such system comprises a source of electromagnetic radiation arranged to pass a beam of radiation through the exhaust plume of a motor vehicle as the motor vehicle passes by the system, and one or more detectors arranged to receive the radiation after it passes through the exhaust plume of the vehicle. A filter may be associated with one or more detectors to enable the detector to determine the intensity of electromagnetic radiation having a particular wavelength or range of wavelengths. The wavelengths may be conveniently selected to correspond to wavelengths absorbed by molecular species of interest in an exhaust plume (e.g., HC, CO, CO2, NOx, or other molecular species). The one or more detector output voltages that represent the intensity of the electromagnetic (em) radiation measured by that detector. The voltages are input to a processor. The processor calculates the difference between the known intensity of the light source and the intensity detected by the detectors to determine the amount of absorption by particular molecular species (based on predetermined wavelengths associated with that species). Based on the measured absorption(s), the concentration of one or more molecular species in the emissions may be determined in a known manner. For various reasons, inaccuracies can occur when remotely sensing emissions.
Some remote emission sensing systems do not have the capability to detect NOx. Other systems detect NOx, but suffer from various drawbacks. One problem is that when detecting the NOx concentration present in an exhaust plume, the presence of ambient NOx can adversely affect the accuracy of the detected concentration. For example, if two cars pass a test station within a relatively short time period, NOx emissions from the first car may linger and be mixed with the exhaust plume of the second car thereby skewing the measurement of NOx concentration of the second car. Other sources of ambient NOx may lead to a similar result.
A second problem arises, due to variations in light source intensity. Generally, to detect the NOx concentration in an exhaust plume, the output of a detector adapted to determine the amount of absorption of the light beam due to the presence of NOx is compared to a value indicative of the intensity of the light source, with the difference representing the amount of absorption due to the presence of NOx.
Typically, a standard value is used for the light source intensity. However, variations in the actual intensity of the source can cause inaccuracies in the detected amount of NOx. A third problem arises due to the presence of noise. Other problems and drawbacks exist.